1. Field of the Invention
The present invention relates to a statically and dynamically pressure balanced three-way fluid control valve.
2. Description of Related Art
Control valves are typically used to control the flow of fluid in a hydraulic or pneumatic system. For example, control valves are commonly used to control the flow of fluid into and out of a hydraulic cylinder. FIG. 1 shows a conventional 3-way control valve that has a cylinder port (C) which can be coupled to either a return port (R) or a source port (S) by the outer groove of a spool. The spool is moved between a first position and a second position by a pair of solenoids. When the spool is in the first position, the cylinder port of the valve is coupled to the return port. When the spool is in the second position, the cylinder port is in fluid communication with the source port of the valve.
Although spool valves are ideally designed to prevent fluid from flowing into the inner valve chamber, machine tolerances, wear and other factors may result in a leakage of fluid into the spaces between the valve housing and the ends of the spool. A build up of fluid within the inner chamber of the housing may create a hydrostatic pressure that counteracts the movement of the spool. The counteracting hydrostatic pressure may slow down the response time of the valve. Additionally, the hydrostatic pressure may cause an improper seating of the spool. An improper spool seating will decrease the area between the spool channel and the port opening, resulting in an increase in the fluid resistance of the valve. It would be desirable to provide a spool valve that is statically pressure balanced to significantly reduce the effects of fluid leaking into the inner chamber of the valve.
The fluid that flows through the ports and the outer groove of the spool has a pressure and associated momentum which applies a force to the spool valve. As shown in FIG. 1, the force of the fluid may be directed in a direction opposite to the movement of the spool. The fluid force will counteract the pull of the solenoid. Relatively large counteracting fluid forces may decrease the response time of the value. Additionally, the fluid force may require larger solenoids to move the spool. Larger solenoids increase the size of the valve. It would be desirable to have a dynamically pressure balanced spool valve that is not significantly affected by the forces of fluid flowing through the ports of the valve.